The invention relates to epoxy resin compositions. In a specific aspect, the invention relates to the stabilization against filler settling of an epoxy resin composition containing a particulate filler.
Epoxy resins are used as thermosettable components of formulations for encapsulating electrical components. In this application, the epoxy resin is conventionally used in combination with a solid particulate filler material, such as alumina trihydrate, tabular alumina, calcium carbonate, silica and the like. The filler not only lowers the cost of the encapsulating material, but also provides an encapsulating formulation having improved modulus and lower coefficient of thermal expansion than the unfilled epoxy formulation. Filled epoxy encapsulation formulations, however, suffer from the problem of reduced shelf life as a result of filler settling upon storage. The filler material settles to the bottom of the container and forms a hard means which is difficult to redisperse through the formulation. Tabular alumina and alumina trihydrate, which are popular filler materials, are particularly difficult to maintain in dispersion. The problem of filler settling necessitates either preparation of the encapsulation formulation at or near the time of application, which may be timeconsuming and inconvenient for end-users, or the risk of using a formulation from which a portion of the filler has settled, which can result in compromised encapsulation properties. Additives, such as 1,2,3-trihydroxy propane, for example, have been used to inhibit filler settling, but have been found ineffective in stabilizing alumina trihydrate filler or have the liability of unacceptably increasing the viscosity of the formulation. Furthermore, some additives are effective only if the filler is added to the epoxy-additive formulation immediately after the formulation is prepared, but are not effective if the epoxy-additive formulation is stored for even a few days before filler addition.
It is therefore an object of the invention to provide filled epoxy resin formulations which have lengthed shelf life and improved resistance to filler settling during storage. In one aspect, it is an object of the invention to provide a method for settle inhibition for epoxy formulations which does not unacceptably increase the formulation viscosity.